The present invention is concerned with liquid crystalline cyclohexane derivatives and with dielectric compositions containing them.
To an increasing extent, the properties of nematic or nematic-cholesteric liquid crystalline materials are being utilized for electro-optical indicating elements. This utility derives from the fact that the optical properties of these materials such as light scattering, birefringence, reflecting power or color, change under the influence of electric fields. The function of such indicator elements thereby depends, for example, upon the phenomenon of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transition.
For the technical use of these effects in electronic elements, liquid crystalline materials which must satisfy a plurality of requirements are needed. Especially important is a chemical stability to moisture, air and physical influences, such as heat, infra-red, visible and ultra-violet radiation and direct and alternating electric fields. Furthermore, there is required a liquid crystalline mesophase in the temperature range of at least +10.degree. C. to +60.degree. C. and preferably of 0.degree. C. to 60.degree. C., and a low viscosity at ambient temperature, which should preferably be not more than 70 cP. Finally, they should not exhibit an inherent absorption of visible light, i.e., they must be colorless.
A number of liquid crystalline compounds is already known. These satisfy the stability requirements demanded of dielectrics for use in electronic display elements and are also colorless. These include, in particular, the p,p'-disubstituted benzoic acid phenyl esters described in U.S. Pat. No. 4,002,670 and the p,p'-disubstituted biphenyl derivatives described in U.S. Pat. No. 3,947,375. In both of these classes of compounds, as well as in other known series of compounds having a liquid crystalline mesophase, there are no individual compounds which form a liquid crystalline nematic mesophase in the required temperature range of 10.degree. C. to 60.degree. C. Therefore, as a rule, mixtures of two or more compounds are produced in order to obtain materials which can be used as liquid crystalline dielectrics. For this purpose, it is customary to mix at least two compounds, one with a low melting and clear point, the other with a distinctly higher melting and clear point. A mixture is thus normally obtained, the melting point of which is below that of the lower melting component, whereas the clear point lies between the clear points of the components. However, optimal dielectrics cannot be produced in this way since the components with the high melting and clear points almost always impart a high viscosity to the mixtures. Consequently, the switch times of the electro-optical indicator elements produced therewith are prolonged in an undesirable manner.